Summary: c-Myc has powerful effects on proliferation and apoptosis and its activation causes many types of tumors, but the mechanisms that mediate these diverse functions remain enigmatic. We have focused on the role of novel c-Myc-interacting proteins on these distinct c-Myc functions. We have shown that the tumor suppressor ARF binds to c-Myc directly and inhibits c-Myc-induced hyperproliferation and transformation, but is required for c-Myc-mediated apoptosis in fibroblasts independently of p53. In addition, ARF inhibits canonical c-Myc transcriptional activity, but induces a novel noncanonical transcriptional mechanism. Furthermore, we have shown that one of the new potential noncanonical c-Myc target genes is required for c-Myc-induced apoptosis, thus suggesting that it may mediate ARF-dependent c-Myc-mediated apoptosis. In addition, we have found that the ability of c-Myc to induce apoptosis is controlled by a specific post-translational modification of c-Myc, which is blocked by ARF binding. Our hypothesis is that a major mechanism by which c-Myc induces apoptosis is through an ARF-dependent induction of specific noncanonical target gene expression, which then mediates the expression of proapoptotic genes, such as Bim. Furthermore, this noncanonical mechanism is dependent on the inhibition of a c-Myc post-translational modification caused by ARF. To test this hypothesis, we propose to 1) Investigate the mechanism regulating ARF-dependent c-Myc-induced apoptosis, 2) Investigate the noncanonical regulation of the transcription factor we identified as a novel ARF- dependent c-Myc regulated gene that mediates c-Myc-induced apoptosis, 3) Determine the biological significance of these novel findings in c-Myc-induced apoptosis and lymphogenesis. These studies will have major implications for understanding the function of c-Myc in apoptosis and in the treatment of cancer.